Long noncoding RNAs (lncRNAs) are enigmatic transcripts of the genome defined as noncoding RNAs over 200 bp in length. Although function has been ascribed to some lncRNAs, most are expressed at low levels, and their function, or even whether they have a specific function, is unknown. By examining the transcription of mammalian lncRNAs, focusing on comparisons between lncRNAs found in mice and humans, Managadze et al. set out to determine how lncRNAs evolve. Calculation of the degree of evolution of these lncRNAs and examination of their relative expression levels revealed a negative correlation between evolutionary rate and expression level. This is similar to what has been observed with protein-coding genes, and like protein-coding genes, nonsynonymous mutations were observed in lncRNAs; that is, mutations predicted to change the amino acid encoded by the codon. No significant correlation was observed between the evolutionary rate of lncRNAs and their predicted secondary structure. On the basis of these findings, the authors conclude that lncRNAs are subject to weak purifying selection and thus are probably functional.